Science News - April 24, 2010

mountain has been pumping out debris flow after debris flow,” says Portland State University’s Burns.

The material in those slumps comes
from two main sources, he notes. One
is the immense volume of fractured
rock that fell back to Earth after being
hurled skyward in volcanic plumes
during the May 18 and subsequent
eruptions. The other is the halo of gla-
cier-scraped bits left behind as the
peak’s ice masses— which somewhat
stabilized the material — retreated
upslope, both in the wake of the erup-
tion and as climate has warmed in
recent years. “When you climb the
mountain, the last hour or hour and a
half is two steps up, one step down, two
steps up, one step down,” Burns says.
“This stuff is two or three feet thick,
and that’s the type of material that’s
being mobilized.”
Prodigious sources of loose glacial
sediment can be found on all Cascade
volcanoes, Burns reported in Portland
last October at the annual meeting of the
Geological Society of America. “You have
the material, you have the steep slopes,
you have no vegetation to hold the mate-
rial in place, and all you have to do is add
water,” he says.

The landslide risk from that material will only increase if the climate gets warmer in coming decades, he speculates. Retreating glaciers have already exposed immense quantities of loose sediment that had accumulated on the volcano’s flanks before the eruption, and a shorter snow season means that the material will be exposed to erosion-causing rains for more time each year.

Beginning this summer, Burns and a few of his graduate students will be surveying Mount St. Helens, mapping the volumes and locations of its loose material and then creating a map depicting the risks of landslides. “It’s going to be a lot more extensive than we’d thought before,” he notes.

Researchers will document the recovery of the terrain around Mount St. Helens for generations to come as ecological succession gradually restores the dense old-growth forests that

Even after 30 years, scientists are gleaning fresh insights from images taken during Mount st. Helens’ latest major eruption. new studies of photos and video shot that day may help volcanologists predict whether an ash plume rising into the stratosphere might be on the verge of collapsing back to Earth in a rain of debris.

From 11 a.m. to about 12: 15 p.m. on the day of the 1980 eruption, Mount st. Helens spewed up to 13,000 metric tons of hot gases and rock each second. During that interval the plume over the peak could pull in enough air to be fully buoyant, like smoke going up a chimney. images taken that morning show that the swirling eddies around the edges of the plume measured about 560 meters across.

But much of the plume collapsed from

12: 15 p.m. to around 4: 25 p.m., as the volcano’s vent widened and mass flow through the crater more than tripled. instead of rocketing skyward, pyroclastic flows of hot gas and rock rolled down the mountain’s flanks. During this phase the ash plume’s eddies measured, on average, only 370 meters across. Researchers speculate that the plume collapsed because these smaller, more tightly spaced swirls couldn’t pull in enough air to keep the rising pillar fully buoyant.

“the real trick for an ash plume is to
incorporate enough air to become buoyant
before it loses its upward momentum and
collapses,” says James E. Gardner, a geolo-
gist at the University of texas at Austin.
now, for the first time, recent analyses of
images captured that day provide vital clues about what happens inside a vol-
cano’s thick ash plume, he and colleague Benjamin J. Andrews report in the
october 2009 Geology.
Although the May 18 eruption is one of the best documented eruptions
in history, scientists don’t have images showing the ash plume’s transition,
says Gardner. so, he notes, it’s not possible to discern how quickly that
transition took place or whether the plume displayed advance warning of its
impending collapse.
it’s tantalizing to think that scientists could spot early indications that
a plume was on the verge of falling back to Earth, says Gardner. And even
though data from Mount st. Helens can’t answer that question, he notes, his
team’s findings nearly 30 years after the explosion open up new avenues for
studies of future eruptions. — Sid Perkins

Mount St. Helens’ 1980 erup-
tion was the largest ever seen
in the lower 48 states.

covered the area before the eruption. And though the chances of another major eruption during that time are slim, someday Mount St. Helens will explode again, smothering the landscape in a mantle of ash and beginning the process anew. s